1. Field of the Invention
This invention relates to a process for working up hydrolysis residues obtained in the hydrolysis of solids-containing polysilane sludges from the synthesis of organochlorosilanes by the direct process. More particularly, the invention relates to the working up of hydrolysis residues from the synthesis of methylchlorosilanes.
2. Background Information
Organochlorosilanes and, in particular, methylchlorosilanes are used as starting materials for the production of silicones which are widely used, for example, as rubbers, jointing compounds, oils, structural sealants, etc. Dimethyldichlorosilane is required in particular for the production of methylchlorosilanes, being obtained in high yields when the direct reaction of silicon with methylene chloride is catalyzed by copper or copper compounds. The process is described in principle in U.S. Pat. No. 2,380,995. The industrial production of methylchlorosilanes by this process is carried out worldwide, the reaction normally being carried out in continuous fluidized-bed reactors.
Where the direct process is carried out in fluidized-bed reactors, the very fine fractions of silicon, catalyst and unreacted contact material are continuously discharged together with the reaction product, the crude silane mixture, and unreacted methyl chloride.
These dust-like fines are often collected together with the highest-boiling reaction products (Bp.sub.760 &gt;160.degree. C.) in a so-called sludge vessel followed by a washing tower. The temperature of the vessel, which is under the usual excess pressure of 1.5-10 bar, is generally adjusted so that the mixture of solids and condensed fractions is kept sufficiently thinly liquid to facilitate discharge from the vessel.
According to DE-PS 2 362 494, the contents of the vessel may be expanded in a stirred container preferably kept at normal pressure and the distillable components are removed from the mixture by heating. The contents of the vessel are then generally hydrolyzed.
The hydrolysis itself may be carried out in a down-pipe, as described in DE-PS 2 362 494. The disadvantage is that, in view of the short contact times, hydrolysis is often incomplete and large quantities of water are consumed.
U.S. Pat. No. 4,221,691 describes a hydrolysis process in which the unpleasant property that the hydrolyzates have of sticking is prevented by the addition of mineral oil. However, since the hydrolyzates are regarded as worthless and are dumped, the additional organic pollution they cause is a disadvantage.
It is known from U.S. Pat. No. 4,408,030 that the problems of sticking can also be overcome by maintaining a minimum chlorine content, although technically this is difficult to do.
In all the processes mentioned above, a hydrochloric acid suspension is formed in which the more or less solid hydrolyzate is regarded as worthless and has to be dumped. However, the hydrolyzates are not without problems because they generally contain 2 to 10% predominantly metallic copper which can be partly eluted from the dumped hydrolyzate, thus endangering the ground water. In addition, most of the hydrolyzates obtained are vulnerable to oxidation and, in some case, even show a tendency to ignite spontaneously so that they cannot be safely dumped.
Now, U.S. Pat. No. 4,758,352 describes a process in which the hydrolysis is carried out in water or a heavily diluted hydrochloric acid in a stirred container which is equipped with a high-speed disk stirrer, but not with baffles so that a vortex into which the material to be hydrolyzed is introduced can form. The preferred temperature is between 60.degree. and 90.degree. C. A suspension of finely divided solid hydrolyzates, in which more than 90% of the solid particles are smaller than 5 mm in diameter, is thus obtained.
The suspensions thus obtained are then oxidized with oxygen-containing gases which, according to the invention cited above, is preferably done with technically pure oxygen under a pressure above atmospheric pressure.
On completion of oxidation, solids and copper-containing liquid are separated from one another.
The process according to U.S. Pat. No. 4,758,352 gives a disposable, compact, non-gasing solid with no elutable heavy metals which is thermally inert in the context of the invention and may thus be safely dumped.